The present invention relates to the bromination of styrenic resins, and is especially directed to the ring bromination of polystyrene. Brominated styrenic resins are used as flame retardants in polymer resin formulations.
Brominated polymers of alkenyl aromatic compounds are useful as flame retardants for thermoplastic resins. There are many known methods for the preparation of ring-brominated polymers of alkenyl aromatic compounds, with most attention directed to the production of brominated polystyrene. Early methods created products wherein many of the aliphatically bonded hydrogen atoms were replaced by bromine atoms. Such products are unsuitable for a number of end uses, as they evolve hydrogen bromide at temperatures of about 180.degree. C. to 250.degree. C. Even when they contain only very small amounts of aliphatically bonded bromine, they develop a dark discoloration at temperatures above 200.degree. C.
The typical procedure for brominating polystyrenes involves dissolving polystyrene having a molecular weight in the range from 100,000 to 250,000 in a chlorinated hydrocarbon solvent, dispersing a Lewis acid catalyst such as iron chloride or aluminum chloride into the solution, and then gradually adding bromine to the solution. Unfortunately, the process is quite slow. The bromination of one gram mole of polystyrene with two gram moles of bromine requires 31/2 to 4 hours. Increasing the ratio of bromine to polystyrene slightly requires substantially longer reaction times. At such higher ratios, the bromination of 0.75 gram moles of polystyrene with 1.87 gram moles of bromine requires 5 hours.
Yet another deficiency with the standard process is a tendency for the polystyrene to be incrementally polymerized. Very high molecular weight chunks of polystyrene tend to build up around the catalyst particles. These undesirable particulate materials have to be filtered out before the brominated polystyrene can be used.
When brominated styrenic polymers are employed as flame retardants in thermoplastics, their color is of great interest to the compounder of thermoplastic materials. The compounder desires a white-colored brominated polymer so that the color of the thermoplastic article, or part, into which the brominated aromatic polymer is incorporated is not affected. It would be desirable to have a brominated polymer of an alkenyl aromatic compound of high molecular weight with a white appearance as manufactured and, equally importantly, which can be produced more cheaply and efficiently.